Re: [PATCH net 4/4] net: ti: icssg: Fix XSK zero copy TX during application wakeup

[Jakub Kicinski <kuba@kernel.org>]

On Tue, 16 Jun 2026 16:41:00 +0530 Meghana Malladi wrote:
> On 6/16/26 04:51, Jakub Kicinski wrote:
> > On Fri, 12 Jun 2026 00:27:44 +0530 Meghana Malladi wrote:  
> >> @@ -169,9 +169,6 @@ static int emac_xsk_xmit_zc(struct prueth_emac *emac,
> >>   
> >>   		num_tx++;
> >>   	}
> >> -
> >> -	xsk_tx_release(tx_chn->xsk_pool);
> >> -	return num_tx;  
> > 
> > Why are you deleting this?
> >   
> 
> xsk_sendmsg() also calls this without an rcu-lock when transmitting the 
> packets if the xmit was successful, so I was assuming it is not required 
> and I removed this.

I think you still need it. Besides, seems like a separate cleanup.

> >>   void prueth_xmit_free(struct prueth_tx_chn *tx_chn,
> >> @@ -279,9 +276,6 @@ int emac_tx_complete_packets(struct prueth_emac *emac, int chn,
> >>   		num_tx++;
> >>   	}
> >>   
> >> -	if (!num_tx)
> >> -		return 0;  
> > 
> > Does something prevent us from running all this code if budget is 0?
> > If budget is 0 we can complete normal Tx with skbs but we must
> > not touch any AF-XDP related state.
> 
> Can you elaborate more, I couldn't interpret your comment here

netpoll may call napi from any context, including from IRQ.
It uses budget of 0 to indicate that it's trying to only reap tx
completions, without doing any Rx or XDP work. XDPs can't be called
from IRQ context.

> >>   	netif_txq = netdev_get_tx_queue(ndev, chn);
> >>   	netdev_tx_completed_queue(netif_txq, num_tx, total_bytes);
> >>   
> >> @@ -306,7 +300,9 @@ int emac_tx_complete_packets(struct prueth_emac *emac, int chn,
> >>   
> >>   		netif_txq = netdev_get_tx_queue(ndev, chn);
> >>   		txq_trans_cond_update(netif_txq);  
> > 
> > This looks misplaced, now we will hit it even if we didn't complete
> > or submit any Tx.
> 
> This code needs to be hit for packet transmission in zero copy mode.
> emac_xsk_xmit_zc() submits the packets to the DMA in NAPI context,
> when application wakes up the driver and triggers NAPI. Once DMA 
> transfer is done, irq gets triggered NAPI gets called which will handle 
> the tx packet completion + submit next Tx batch packets to the DMA.
> 
> if (tx_chn->xsk_pool) -> check ensure this hits and runs for zero copy 
> only. Also above check (!num_tx) returns early during the application 
> wakeup (where budget is zero), hence it is removed.

I'm commenting on txq_trans_cond_update(), you're calling it
effectively on every NAPI call when XSK is bound, whether
Tx is making progress or not.